1. Field of the Invention
The present invention relates in general to fuel detecting devices which detect a particular component in a mixed fuel, and more particularly to fuel detecting devices of a type which detects the concentration of alcohol (viz., methanol, ethanol or the like) in a mixture of gasoline and alcohol.
2. Description of the Prior Art
For dealing with the energy crisis and air polution problems, it has been proposed to use as the fuel of automotive internal combustion engine a mixed fuel, such as a mixture of gasoline and alcohol.
When such mixed fuel is actually used as the fuel for the engine, it becomes necessary to detect the concentration of alcohol in the mixed fuel in order to appropriately control the substantial air-fuel ratio of the combustible mixture which is fed to the engine.
One of the known devices for detecting the alcohol concentration in the mixed fuel is disclosed in NISSAN AUTOMOTIVE TECHNICAL REPORT (issued from NISSAN MOTOR CO., LTD. in December, 1985). In this device, a so-called "capacitance type alcohol sensor" is used, which comprises a pair of spaced electrode plates which are to be submerged in the mixed fuel. In operation, the capacitance established between the two electrode plates in the fuel is measured to obtain the dielectric constant of the fuel, and the alcohol concentration is derived from the dielectric constant.
In order to clarify the aim of the present invention, the conventional device disclosed by the REPORT will be outlined in the following with reference to FIG. 15 of the accompanying drawings.
As is seen from FIG. 15, the device comprises the capacitance type alcohol sensor 1 which has a pair of spaced electrode plates submerged in alcohol-mixed fuel in a fuel tank 2. The change in the capacitance established between the two electrode plates is processed by a detecting circuit 3 in which the capacitance change is converted into a current change or signal frequency change for calculating the alcohol concentration in the mixed fuel. The detecting circuit 3 includes a correction unit by which the current change or signal frequency change is corrected in accordance with the temperature of the mixed fuel. An indicating device 4 receives the corrected signal (viz., current change or signal frequency change) from the detecting circuit 3 and processes the signal to give a visual indication of the alcohol concentration.
However, due to its inherent construction, the above-mentioned device has the following drawbacks.
First, since the capacitance sensed by the alcohol sensor is directly fed to the detecting circuit 3, it is very difficult to pick up, from the sensed capacitance information, only the component which represents the alcohol concentration.
Furthermore, in the measuring method employed in the device, it is very difficult to reduce or minimize unavoidable errors in the signal from the detecting circuit 3, the errors being particularly caused by the ambient temperature (that is, the engine room temperature in which the detecting circuit is installed). This means that the device is not suitable for a motor vehicle which is subjected to a wide ranging movement from a cold area to a hot area and vice versa.